One metallurgical furnace wherein blast air is used is a blast furnace. A blast furnace is kept in operation by blowing preheated air into it through a number of tuyeres. The air may be enriched with oxygen; also liquid or gaseous hydrocarbons, or pulverized coal may be added to the blast air as desired.
An elongated bubble-shaped space, called a raceway, exists in front of the nose of each tuyere. These raceways are diagramatically depicted in FIGS. 3 and 5 of United Kingdom Pat. No. 400,793. However, those skilled in the art will recognize that the raceways in a given furnace are not necessarily equal to each other in depth of penetration into the furnace nor in peripheral scope.
Combustion takes place in the raceways where the coke of the burden, and the additives, if used, which are introduced through the top of the furnace, burn to CO.sub.2 and H.sub.2 O, which, in turn, are further reduced to CO and H.sub.2. The CO and H.sub.2 are then utilized as reducing agents to reduce the metallic materials of the burden.
It is generally recognized by those skilled in the art that the size of the raceways and their shape depend on the volume, pressure and temperature of the blast, as well as the cross-sectional size of the tuyere mouth. The degree of penetration of the blast into the burden in the furnace, namely, the length of the raceway from the tuyere mouth toward the center of the furnace, depends on three factors:
(1) the mass of the fluid passing through the tuyere; PA1 (2) the flow velocity of the fluid jet leaving the tuyere mouth; and PA1 (3) the magnitude of the thrust of the jet which is greatest whenever parallel flow of the fluid current is insured. PA1 ". . . there is arranged in the tuyere of the shaft furnace, instead of the deflecting body, a perforated body which preferably has the shape of a smaller tuyere, in such manner that it may be wholly or partially inserted in the axial direction of the main tuyere, into the interior of the hearth. With such a shifting of the hollow insert body, a blast inlet position in the hearth and consequently the oxidation zone, is shifted during the working of the shaft furnace to any desired degree." PA1 ". . . the insert hollow body 6 is moved out of the tuyere 2 into the interior of the hearth by as much as the oxidation zones are to be moved forward. If the body 6 is fully drawn back into the tuyere 2, then the oxidation zone in front of the tuyere again receives that position which is given by the position of the tuyere itself." PA1 "That the deflecting body should always be centrally arranged in the tuyere 2 is important as otherwise a tuyere current is produced which cannot be exactly controlled."
Blast fluid velocities also significantly affect the temperature distribution within the raceways. With increasing jet velocities, the region of highest temperature moves away from the tuyere nose and the temperature at the periphery of the raceways falls. The opposite effect can be observed when natural gas is used as an additive to the blast fluid. A result of unevenly distributed temperature in the raceways are erratic currents within the blast furnace which cause the burden to descend unevenly, thus disturbing stable furnace operations. This tendency toward uneven distribution of burden descent is enhanced by uneven or disproportionate tuyere operation, in relation to the other tuyeres within the blast furnace, which increases the incidence of erratic and undesirable currents.
Recent reports on blast fluid distribution among the tuyeres of large blast furnaces indicate that those tuyeres farthest from the blast fluid entrance into the manifold bustle pipe receive more mass of fluid than those nearer to the hot blast main entrance to the bustle pipe. The total blast is unevenly distributed among the tuyeres of equal mouth size cross-sectionally, resulting in the furnace working one-sidedly causing impaired production of pig iron.
Those skilled in the art will recognize that there are certain general steps that can be taken to obtain optimum production in large blast furnaces. First of all, good burden size is needed. Secondly, the top pressure can be increased to obtain additional production. Thirdly, oxygen enrichment can be added to further increase production. Fourthly, tuyere jet velocities should be maintained within narrow ranges relative to each other and they must be corrected for any change in blast conditions. Finally, deep penetration of the blast fluid jet into the burden should be insured.
Various means have heretofore been proposed to control the fluid flow both within the tuyere and as it leaves the tuyere mouth. It was suggested to employ transversally movable plates, cooperating with damper plates, for varying the inlet and outlet area of the tuyere. An outlet control similar to that suggested is disclosed in U.S. Pat. No. 296,225 wherein a water cooled gate is movable to block the flow of blast fluid to varying degrees depending on the position of the gate in relation to the tuyere mouth. Inserts have been suggested for placement inside the tuyere nose to reduce the cross-sectional area of the tuyere mouth, the insert being knocked out by a rod inserted through the peephole when it is desired to increase that area. Such a system is disclosed in U.S. Pat. No. 2,087,842. It has been proposed to provide an apparatus comprising a water-cooled ring disposed within and coaxially of a conical tuyere, the outside diameter of the ring being equal to the smallest internal diameter of the conical nozzle, the tuyere mouth. Such an apparatus is disclosed by U.S. Pat. No. 636,239. As mentioned, it has also been proposed to control the blast flow velocity as it leaves the tuyere mouth. Specifically, such a method is accomplished by deflection means extending beyond the tuyere mouth into the blast furnace hearth. Apparatus to effect this method is disclosed in United Kingdom Pat. No. 400,793. And, there is a "streamlined" body mentioned in the French Patent Letter No. 1,009,336, which body is a rigidly mounted vaporizer employed to add atomized water droplets to the blast fluid current; it is positioned inside of a tuyere, but it is not used for varying blast jet velocities.
U.S. Pat. No. 296,225 discloses various means of externally controlling a sliding stopper. The stopper functions by sliding transversally across the tuyere mouth similar to the operation of a gate valve. All of the external means for operating the stopper produce a transverse motion of the stopper which is opposed to the axis of the tuyere. The tuyere disclosed is of a special design, radically different from the tuyeres normally used in blast furnaces. This novel tuyere has an upper section which is out of the direct flow of the hot blast fluid, providing a convenient arrangement within and through which means to operate the stopper can be placed. However, this upper section provides an ideal means to create undesirable turbulance and unbalanced gas flow. There is no way disclosed or described by which the external operation means could be utilized in a cylindrical cross-section tuyere where no additional space is provided within the tuyere for the movement of the disclosed external operating means.
U.S. Pat. No. 636,239 discloses a hollow ring adapted to be displaced substantially axially within a tuyere until it fills the tuyere mouth and leaves only the central opening of the ring for the passage of the blast; then, by pulling the ring back, a varying amount of free space is left between the ring and the nozzle. As indicated by phantom outline in FIG. 1 of this patent, the ring drops below the longitudinal axis of the tuyere when withdrawn from the tuyere nose, resulting in an unbalanced gas flow.
United Kingdom Pat. No. 400,793 discloses, in item 6 of FIG. 6, a symetrical cylindrical body with a uniform wall thickness, the outer surface being coaxial with the bore. The walls are hollow, providing a coolant passageway. The specification of this patent states that the cylindrical body also takes the shape of a blast tuyere. The indication is that, in the shape of a blast tuyere, the cylindrical body would more closely resemble the preferred embodiment of the invention shown as item 1 in FIG. 1 therein; the positioning would be such that the smaller end of the blast tuyere shape would be directed away from the center of the blast furnace to enable the required deflection of the blast fluid in a manner similar to that shown in FIG. 4.
The manner in which the cylindrical body function is disclosed in United Kingdom Pat. No. 400,793 at page 2, lines 40 to 52 as follows:
The specification further details the operation of the hollow insert at page 3, lines 3 to 11:
United Kingdom Pat. No. 400,793 also teaches a most significant point at page 2, lines 90 to 94:
This language points out a critical deficiency found in the apparatus disclosed in U.S. Pat. No. 636,239.
Those skilled in the art are aware that modern tuyere arrangement design includes a horizontal blowpipe which carries the hot blast from the tuyere stock to the tuyere proper. The blowpipe has spherically machined ends that fit tightly into the machined end of the tuyere proper and the tuyere stock to give the arrangement a tight fit, even though the tuyere proper and the tuyere stock may be misaligned. The blowpipe is held in place by pressure from the tuyere stock which, in turn, is held tightly against one end of the blowpipe by a heavy spring and rod device called the bridle. The bridle is attached to the hearth jacket, through which the tuyere extends, and allows limited motion between and misalignment of the central axis of the tuyere stock, blowpipe and tuyere proper caused by inexact construction and by thermal movement of the furnace shell. The movement and misalignment so caused eliminates the ability to centrally position and arrange an insert within a tuyere proper, which insert is supported from means extending from the peep sight position on the tuyere stock as disclosed in both U.S. Pat. No. 636,239 and United Kingdom Pat. No. 400,793. Both of these patents include means disposed inside of and extending through the blowpipe to actuate the longitudinal travel of inserted bodies. Such arrangements interfere with the smooth supply of additives through the blowpipes; also, the need for replacement of the blowpipes and/or the tuyere causes difficulties in realignment.
The present invention employs actuators disposed fully outside the blowpipe and not extending therethrough, and the brackets holding the independent inserts are guided parallel to the tuyere's axis within the tuyere proper, securing thereby always a central position of the movable inserts. The principal objective of the present invention is to provide means for obtaining and securing optimum thrust for any given blast condition by achieving an exactly controllable concentric blast flow pattern simultaneously for a set of individual tuyeres. The principal objective being realized, it is possible to correct unstable working of the furnace, to equalize the uneven blast distribution, and to compensate for the changing blast conditions by enabling the furnace operator to set the tuyere jet velocities individually during furnace operation.